Artificial respiration of patients is necessary for various medical applications, e.g., during surgery. Respirators are used for the artificial respiration of patients and can additionally also be used for anesthesia as anesthesia apparatuses with an anesthetic reflector and anesthetic dispenser. The expiration gas expired by the patient can be reused again at least partly as inspiration gas in some respirators, i.e., these represent a rebreathing system with a breathing air circulation system. A gas delivery means, which ensures that the gas used up by the patient, for example, oxygen, is added again to the inspiration gas, is present in the respirator with the breathing air circulation system.
Measurement of a volume flow of one or more gases is necessary in respirators or anesthesia apparatuses. It is necessary, in particular, in a gas mixer of an anesthesia apparatus, to measure the volume flows of the different gases fed to the gas mixer, for example, oxygen, laughing gas and air. The gas, whose volume flow is to be measured, is sent through a flow channel with a first pressure-measuring point and a second pressure-measuring point. The flow channel has a cross section contraction between the first and second pressure-measuring points. Increased friction, which leads to a loss of energy of the gas, which can be measured as a pressure drop, will develop in the cross section contraction due to the increased velocity. The volume flow can be calculated from the measured pressure difference. Additional parameters, for example, the absolute pressure, temperature and geometric properties of the flow channel as a measuring channel, are necessary here. A U-shaped channel is used as a flow channel in the direction of flow of the gas to be measured. The U-shaped channel comprises three holes arranged at right angles in relation to one another in an aluminum block. As an alternative, a sintered body or a gap may also be used to contract the cross section. The manufacture of the U-shaped channel with the three holes arranged at right angles in relation to one another is complicated by means of injection molding with an injection die because slides are necessary for this. Narrow gaps can also be prepared with difficulty only by means of injection molding as a cross section contraction, e.g., as a diaphragm, because thin die structures, which are highly unstable, are to be prepared for this. Long flow channels are also disadvantageous because of the large space required at a respirator or anesthesia apparatus.